The present invention relates to a semiconductor element and a liquid crystal display, and in particular, it relates to a semiconductor element, suitable for use as a thin film semiconductor element, and an active matrix addressing type liquid crystal display using the same.
As display devices for displaying image information and characters for use in the field of office automation equipment and the like, active matrix addressing liquid crystal displays using thin film transistors (hereinafter referred to as TFT) have been known. For this type of conventional liquid crystal display, further improvements in resolution and high quality of display, as well as cost reduction in manufacture, are necessary. In order to solve these problems, it is most important to improve the performance of the thin film transistor, which is a key device to the active matrix addressing liquid crystal display. In this regard, in the Conference Record of International Display Research Conference 1985, page 9, a concept of forming advanced high performance TFTs on a glass substrate was proposed, in which peripheral drive circuits for driving a TFT active matrix are fabricated using TFTs and are integrated with its display unit on the same substrate so as to reduce the cost of manufacture. With provision of advanced high performance peripheral drive circuits integrated on the glass substrate, since other circuits to be mounted externally and their mounting process can be simplified, a substantial reduction in the packaging cost can be expected. However, in order to provide these high performance circuits, development of a more improved, sophisticated high performance TFT is required. In particular, TFTs, which are formed on polycrystalline silicon (hereinafter referred to as poly-Si) and appear to be the most promising TFTs suitable for use in display equipment having integrated peripheral drive circuits, have further technical problems yet to be solved, which problems include the need for improvement of the carrier mobility and a lowering of the threshold voltage Vth.
A method of operation at a lower voltage by lowering the threshold voltage Vth of the transistors formed on an insulator has been proposed, as described in the Technical Digest of International Electron Device Meeting 1994, pages 809-812, in which a so-called dynamic threshold voltage MOSFET (hereinafter referred to as DTMOS) which has a fourth contact, separate from the source and drain, on a semiconductor film acting as a bipolar active layer, and is in contact with the gate electrode, is proposed as a submicron silicon-on-insulator (SOI) device.
In the technique described in the above-mentioned publication, the threshold voltage of the transistors is substantially reduced further to lower the operating voltage of the circuits thereof, thereby lowering the power supply voltage, and thus minimizing the power consumption in the drive circuits. This feature of decreased power consumption achieved by this drive circuit is desirable for use as a drive circuit and device for a liquid crystal display as well. It should be noted, however, that the above-mentioned publication is directed to the provision of logic devices or memory devices that can be operated at a low voltage and a high speed, and so they cannot be applied, as they are, to the drive devices for driving a liquid crystal display.
Namely, the semiconductor device, as described in the aforementioned publication at pages 809-812, is designed to operate effectively with a gate voltage at 0.6 V or less, and so this semiconductor device cannot be applied as it is to a liquid crystal display in which a lower limit of the gate drive voltage is determined by the liquid crystal drive voltage. More particularly, at least xc2x13 V in peak amplitude is required for driving normal liquid crystal materials, thereby a higher voltage than this minimum peak voltage is required as the gate voltage for driving pixel drive transistors in a liquid crystal device. Thus, the semiconductor device described above, which operates at the gate voltage less than 1 V, cannot be applied to a liquid crystal display.
An object of the invention is to provide an active matrix address type liquid crystal display in which the power consumption can be substantially reduced.
Another object of the invention is to provide a small power consumption liquid crystal display having a built-in drive circuit.
Still another object of the invention is to provide a drive circuit for use in a small power consumption liquid crystal display.
According to the invention, a semiconductor element for use as a switching element to be formed in a display area of the liquid crystal display or for use as a drive element to be formed in the area of a drive circuit for driving the display area is comprised of a first electrode, a second electrode, a third electrode and a fourth electrode; a pair of first conductivity type semiconductor layers separate from each other and connected to the second and the third electrodes, respectively; an intrinsic semiconductor layer connected to the pair of first conductivity type semiconductor layers; and a second conductivity type semiconductor layer formed on the intrinsic semiconductor layer; wherein an insulating film is interposed between the first electrode and the intrinsic semiconductor layer; and the fourth electrode is formed on the second conductivity type semiconductor layer which is formed on the intrinsic semiconductor layer.
Preferably, the pair of first conductivity type semiconductor layers and the second conductivity type semiconductor layer are separated from each other by the intrinsic semiconductor layer. Further, according to another aspect of the invention, the intrinsic semiconductor layer extends in lateral directions beyond the insulating layer at least towards the pair of the first conductivity type semiconductor layers on the substrate.
The first electrode and the fourth electrode are electrically connected via a resistance advantageously for wiring the liquid crystal display.
The intrinsic semiconductor layer, the first conductivity type semiconductor layers and the second conductivity type semiconductor layer are preferably formed of a thin film semiconductor made either of silicon, silicon germanium or silicon carbide. Further, they may be made of polycrystalline silicon films as well. In particular, it is preferable to use polycrystalline silicon films in a drive circuit built-in type liquid crystal display.
The semiconductor element of the invention can be designed to have a structure either of a planar type, an inverted staggered type or a positive staggered type. In the case of the inverted staggered type, a first electrode is formed on one of a pair of substrates, an insulating layer is formed on the first electrode, and an intrinsic semiconductor layer is formed on this insulating layer, then a pair of first conductivity type semiconductor layers are formed on the intrinsic semiconductor layer. Further, in the case of the planar type, the second, the third and the fourth electrodes are formed on one of the pair of substrates.
In the cases of both the positive staggered and the inverted staggered types, it is also possible for either one of the pair of the first conductivity type semiconductor layers or the second conductivity be formed by self-alignment utilizing a counterpart""s pattern as its own masking pattern.
The semiconductor element of the invention can be used as complementary Nxe2x88x92type and Pxe2x88x92type semiconductor elements which can be formed in the drive circuit region in the liquid crystal display. More particularly, since a vertical scanning circuit and a picture signal drive circuit are provided in the drive circuit region, the complementary semiconductor element of the invention can be applied to a shift register within these circuits. More specifically, when the semiconductor element of the invention is used in the drive circuit region in a peripheral circuit built-in type liquid crystal display, the power consumption thereof can be reduced effectively and substantially.
According to the invention, an intrinsic semiconductor layer is formed as an i semiconductor layer between the first, the second and the third electrodes. Into this intrinsic semiconductor layer, a current carrier is injected which has a different polarity from the polarity of a current carrier in the impurity semiconductor layers of the first conductivity type which are connected to the second and the third electrodes respectively. According to this arrangement, when the impurity semiconductor layers connected to the second and the third electrodes, respectively, are formed of an nxe2x88x92type semiconductor layer, there is formed a p-i-n junction between a current carrier injection layer or the fourth electrode and the second electrode.
Further, when a voltage applied across the current carrier injection layer, i.e., the fourth electrode and the second electrode, is Vb, the voltage Vb is divided between an i-n junction and the i layer of the p-i-n junction, thereby preventing an excess current (base current) from flowing. Thereby, operation at a low power can be ensured until the gate voltage (to be applied to the first electrode) increases substantially. Namely, if a Pxe2x88x92type or Nxe2x88x92type semiconductor layer is used in place of the intrinsic semiconductor of the invention, a full voltage of Vb is applied to the source junction, and if Vb exceeds a built-in voltage (approximately 0.6 V) of the source junction, an excess current is caused to flow, thereby rapidly increasing the power consumption. In contrast, if an active semiconductor layer is comprised of an intrinsic semiconductor, excess base current will not flow until a substantially higher gate voltage is applied, thereby ensuring operation at a low power. In this case, due to adoption of the p-i-n junction, the degree of drop in the threshold voltage becomes mild, but it is still substantial compared to the conventional elements. Due to this drop in the threshold voltage, it becomes possible to operate the liquid crystal driving elements and associated peripheral circuits at lower voltages, thereby substantially reducing power consumption in active matrix type displays. BRIEF